Paper 100801

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Windows for buildings in hot arid countries

H. Askar, S.D. Probert, W.J. Batty*

Department of Applied Energy, School of Engineering, Cranfield University,

Bedfordshire MK43 0AL, UK

Abstract

Too often, in the last 50 years, the energy-thrift lessons of vernacular architecture have been

forgotten or ignored. In the Middle East, many recently-designed commercial buildings, with

large areas of glazing, incur excessively high electricity-demands to provide energy for the

required air-conditioning plant. One way of reducing the magnitude of this demand is through

better window design. For this purpose, a new form of triple-glazed window is proposed to

facilitate achieving improved thermal comfort within buildings, while simultaneously reducing

the expenditures on purchased energy.# 2001 Elsevier Science Ltd. All rights reserved.

Glossary

Arabesque decoration: Ornamental surface reliefs and motifs, which consist of the

frequent repetition of botanical and/or geometrical shapes: these have been

made of wood, glass and/or ceramics, and are usually highly coloured.

Colonial-style architecture: (i.e. of the period from the late 18th to the early 20th cen-

tury). Such types of architecture were introduced primarily by the English, Por-

tuguese, French and Spanish, when they colonised parts of Asia, South Americaand the Middle East. Although local building materials were employed, the

designs of the buildings reflected, to a significant extent, the previous life styles of

the immigrants, rather than the local vernacular architecture.

Glasses: Their structures consist of complex, continuous networks of silica-oxygen

bonds [1], in which the individual electropositive elements are bound to oxygen

[2]. These non-crystalline solids tend to be transparent and brittle: they melt

over a range of temperature.

Glazing angle : Inclination to the horizontal of the external glazing pane or inner

panel in the proposed glazing system, see figs 15 and 21.

Applied Energy 70 (2001) 77101

www.elsevier.com/locate/apenergy

0306-2619/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

P I I : S 0 3 0 6 - 2 6 1 9 ( 0 1 ) 0 0 0 0 9 - 5

* Corresponding author. Tel.: +44-1234-750111; fax: +44-1234-750728.

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International style: This class of urban architecture, that occurred predominantly in

the period from the late 1920s to the mid-1960s, involved the use of new appli-

cations for steel, glass and reinforced concrete. It tended to show more clearly

each buildings structural elements and services. As the unit prices of fossil fuelswere then so low, the energy-thrift lessons of traditional vernacular architecture

tended to be ignored: hence these buildings depended primarily on the use of

mechanical services (e.g. refrigeration plant) for achieving thermal comfort

within their internal artificial environments.

Organic layout: This phrase is used in architecture and planning to indicate that a

natural growth of an urban settlement had occurred; the expansion being dic-

tated by immediate need, with little or no pre-planning [3].

Shading coefficient: The ratio of the solar-heat gain going through unit area of the

considered type of glass to that for unit area of clear float-glass, 3 mm thick,

under identical applied conditions

Solar altitude: The angle subtended between the direct line to the Sun and the hor-

izontal surface

Solar time: Time based on the apparent angular motion of the Sun across the sky,

with solar noon being the time that the Sun crosses the meridian of the obser-

ver, i.e. the plane passing through the observer and the north and south poles.

Ziggurat: A Babylonian ($30001250 BC) name for a stepped pyramidal structure,

with diminishing stages (i.e. podiums) with height, served by a ceremonial

ramp. A small temple or shrine, with a dome of up to 20 m height, was erected

on top of the uppermost terrace: it was intended to be used for observing thestars or as the location for worshiping gods.

Nomenclature

a, b Coefficients

B, C, E, F, H, Iand P Geometrical parameters (all in m) for the window

assembly see Fig. 2

D Daily diffuse radiation received at the Earths surface:

(kWhm2)

Gmax, Gmean and Gmin For a specific month, the daily maximum, mean andminimum amounts respectively of the incident global

solar radiation at the Earths surface, (kWh m2), where

Gmax=G0(a+b), Gmean=G0(a+b/2) and Gmin=G0a

G0 For a specific month, the daily mean extraterrestrial

solar irradiation incident upon a horizontal plane, i.e.

the amount of insolation before experiencing

attenuation and scattering by the Earths atmosphere,

(kWh m2)

hc Convective heat-transfer coefficient for a gas-filled

vertical cavity within a double-glazed system, seeFig. 18, (Wm2 K1)

78 H. Askar et al. / Applied Energy 70 (2001) 77101

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1. The challenge

In the arid countries of the Middle East, more than 40% of purchased energy isused to heat or cool the interiors of buildings. A small percentage reduction of the

annual fuel-consumption nationally would result in huge financial savings. A weak-

ness in the energy-control design of a building has traditionally been the window,

which during (i) cold weather (e.g. at night), allows heat to escape to the ambient

environment and (ii) daytime, permits insolation to enter the building almost unin-

hibited, so possibly leading to over-heating and discomfort. In modern buildings, it

is common practice to use large windows, and hence high rates of heat transfer

occur through them. However, windows also allow daylight to penetrate into

buildings and so satisfy at least some of the lighting requirements of internal spaces.

Well-designed natural lighting can enhance the appreciation of the indoor environ-ment as experienced by its occupants. Thus, for designers, a conflict often exists in

resolving the light and heat transmission behaviours of glazing systems. Conse-

Ktmax Ktmin: Maximum and minimum daily clearness-indices, i.e. the

ratios of the maximum and of the minimum mean

monthly daily global insolations respectively to themonthly mean daily extraterrestrial insolation (i.e.

Gmax/Go=(a+b) and Gmin/Go=a respectively).

S Number of hours of bright sunshine in one day (i.e.

24 h) averaged over a month

Sd Number of hours per day when clouds obstruct a

view of the Sun, averaged over a month

Smax Estimated astronomical mean number of hours of

maximum sunshine in one day for a particular month

UV Ultra violet

W Width of the cavity in the proposed double-glazingsystem, see Figs. 19 and 20 (m)

x Horizontal displacement from the vertical pane of a

vertical transparent insert, see Figs. 20 and 21, (m)

y Height of a vertical insert in the cavity, as in Figs. 20

and 21 (m)

Inclination to the horizontal of the external glazing or

inner membrane in the proposed glazing system, see

Figs. 15 and 21 (degrees)

1,2 Mean solar azimuth angles for summer and winter

respectively, see Fig. 4 (degrees)

T Temperature difference (C)

Transmissivity

N.B. Daily implies the total value of the considered parameter over a 24-h

period

H. Askar et al. / Applied Energy 70 (2001) 77101 79

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quently, for each location, optimising their designs, as static systems, is desirable so

that the best overall appropriate performance is achieved.

2. Vernacular architecture

Civilisation started in agricultural settlements in Mesopotamia (i.e. present-day

Iraq and the eastern part of Syria) more than 7000 years ago. Subsequently, the

inhabitants of this region evolved from living in villages with cottage-type settlements

to the more complex and versatile city communities. During this development, the

populations continually adapted their modes of living and building styles to accom-

modate to the terrain and climate of the local natural-environments. Buildings and

the urban environment were customised to attenuate the extreme effects of natural

forces and the arid terrain by offsetting the severe adverse impacts of the ambient

conditions [4]. Lifestyles and clothing were adapted as part of the survival and cul-

tural responses to the local environment. Desirable benefits were achieved by adopt-

ing various building and environmental features, some of which are listed below:

. Ensuring that the building was of high thermal-mass reduced the internal

temperature fluctuations arising from diurnal-nocturnal ambient temperature-

variations.

. Sizes and locations of the openings through the outer walls and roof were

optimised with respect to the heat and light transfers through them, and fordefensive reasons.

. Shading by natural means (e.g. via trees) reduced the insolation entering adja-

cent buildings and the effects of wind. This was also achieved in urban envir-

onments with shaded alleys providing some thermal protection.

. Reducing the ratio of the external-surface area to the contained volume of the

building, thereby reduced both (i) the total insolation on its exposed surfaces

(i.e. roof and walls) and (ii) the heat transfer rate per unit surface area per unit

volume of the building.

. The pergola system (i.e. the repetition of shaded and sun-lit zones) inhibits the

rate of heat gain and reduces diffuse reflection to surrounding dwellings.. Narrow streets can behave as cooling ducts by venting away hot dusty air [5].

Increasing the wind-exposed surface areas of the external walls and other

building elements, enhanced the rate of heat loss via winds to the ambient

environment. This effect was achieved by introducing brick reliefs, motifs and

alcoves [3] see Fig. 1.

. The rates of heat transfer through the facades of buildings were reduced by

employing low-thermal-conductivity building materials as well as designs that

incorporated walls with cavities that acted as air ducts for heat-exchange purposes.

. The reflectiveness of a facade with respect to insolation could be increased by

painting it white or utilising glazed brick-facings. The latter were used extensivelyin Mesopotamian temples and palaces and subsequently throughout the entire

Middle East during the era of the Islamic Empire (700!1265 AD).


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The microclimates within the habitable spaces of a building were controlled by:

(i) Walls that were glazed preferentially to admit insolation: where this has to be

inhibited, fewer, smaller or no windows were incorporated in these walls.

(ii) Appropriate location and orientation in order to cool the buildings fabric

and, if advantageous, introduce air currents into the inhabited spaces.

(iii) Employing ducts, wind-towers and shafts to promote this air circulation: the

air entering the building may have been passed over intermittently-wetted cloths,

thereby cooling it via latent heat extraction as well as increasing its humidity.

(iv) Introducing an open-unit living space, i.e. a courtyard, within the building:

this void helps by trapping cooler (i.e. denser) air at night, and facilitates the

persistence of its stratification during the day [5].

(v) Reducing the temperature of the air through evaporation and hence latent-

heat cooling by means of fountains and pools located in the courtyard: this

increases the humidity of the air so providing a less arid environment.

(vi) Having live plants, within the building and courtyard, to absorb CO2. This

promotes their growth via photosynthesis, and helps to stabilise the temperature

locally through the process of transpiration from the foliage: besides providing

shading, the foliage also filters out hot dust and other pollutants from the air.(vii) The building including a vented vault and dome [4], thereby promoting the

occurrence of an air current through its internal environment.

Such features and techniques led to various zones within the building being occu-

pied preferentially at different times of the day and year according to the daily

movements of the Sun and seasonal variations in climatic conditions.

3. Applications of glass

Glass technology started in the Middle East several thousand years ago. Duringthe Stone-age (circa 50,000 BC), naturally-occurring volcanic glass (i.e. obsidian) was

used in cutting tools [6]. Crude glass forms existed in the region (circa 4000 BC) as

Fig. 1. Examples of brick relief, as initially employed during the first Abased era (circa 900 AD).


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glazed cones embedded in mortar with their bases remaining exposed to the out-

doors. These glass cones served as tiling in ancient temples (i.e. the ziggurats of the

Sumerians) and in the famous gate of the city of Babylon. The oldest remaining

glass artefacts manufactured by humans are glass pearls, which were stolen subse-quently from the graves of Egyptian pharoahs, who lived $3500 BC. The Phoeni-

cians (circa 850 BC) were the first to produce artificial glass, on a relatively large

scale (for those times), by burning earthenware, thus combining arenaceous lime-

stone containing sand with natron (sodium carbonate). The clay-table library of the

Assyrian king Assurbanipal (circa 700 BC) contains the oldest remaining recipe for

the formation of glass namely: take 60 parts sand, 180 parts ash from sea plants

and 5 parts of chalk, heat and you get glass [7].

The invention (circa 200 BC) of glass-melting ovens and glass-blowers pipes

revolutionised the manufacture of glass and made possible the production of flat

glass, and window glazing. Subsequently, glass was employed extensively in the

decoration of lanterns as well as for domestic artefacts. The most common type,

namely silica-based glass, had appropriate chemicals added when molten to give it

the required colour. Nevertheless, the use of glass in windows was rare until early in

the Islamic Empire period (i.e. $750850 AD). The production methods, which had

been introduced earlier in the neighbouring Persian and Roman Empires, were then

copied frequently. Glass was used extensively in the palaces of the caliphs and in

mosques. Subsequently, technological interactions ensued with the craftsmen of the

neighbouring Byzantine Empire and the crusaders, as can be seen in the design of

windows in Byzantine palaces and churches (circa the 10th century AD). During theperiod from the 10th to the 12th century AD, many studies were devoted to the

development of glass. Its coloration was achieved by adding metallic salts and oxides

to the molten glass (e.g. gold produces a cranberry tint; cobalt makes blues; silver

creates yellow; whereas gold and copper together can create greens or brick red,

when mixed in the appropriate proportions).

Techniques for constructing stained-glass windows were written down by Theo-

philus, a Greek monk, in about 1100 AD. He stated If you want to assemble a

simple window, first mark out the dimensions of its length and breadth on a wooden

board, then draw the scroll work or anything else that pleases you, and select col-

ours that are to be put in. Cut the glass and fit the pieces together with a gropingiron. Enclose them with lead cams and solder on both sides. Surround with a woo-

den frame, strengthened with nails, and set it up in the place where you wish. The

basis of this method has changed little during the subsequent 900 years.

During the Gothic era (circa 1200 AD), many of the great cathedrals of Europe

introduced stained-glass windows. Churches became taller and lighter, with their

structural weights, especially of the roofs, being supported by columns and flying

buttresses. This allowed the introduction in walls of large openings, which were filled

with stained-glass windows. Simultaneously, the Islamic Empire employed intricate

patterns of stained glasses in motifs and designs; these are referred to as Arabesque

decoration [6]. During the Ottoman era (circa 1500 AD), the designs of buildings didnot differ fundamentally from those of the Arabic era prior to 1260 AD. Never-

theless, during the 16th century, the designs of public buildings became increasingly


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decorated. Good examples of this can be seen in the Al-azhar mosque (in Egypt),

and theAl-qushala building (in Iraq).

During the late 18th to the early 20th centuries, European expatriates developed

what has subsequently been called the colonial style [5]. The associated hybriddesigns introduced elements from the architectures of the expatriates origins, but

adapted them to the more hostile climates of the Middle East. The main aim was to

reduce the heat gained by the buildings from the harsh ambient environments during

daytime. Local building materials with high thermal-storage capacities as well as small

windows well setback into the walls were two of the main features of this colonial-

style architecture. The maximum size of a window was limited during the 18th century

to approximately 0.75m0.75m by the then prevalent crown process glass-manu-

facturing technique. However, each building usually possessed individuality because its

features tended to be dictated by the local weather, as well as geographical and cul-

tural conditions of the host country. This led to the evolution of new types of inner

spaces (i.e. artificial environments) that previously had not existed in indigenous

vernacular buildings. The nineteenth century saw the peak of post-and-lintel con-

struction that permitted larger window openings (i.e. up to 1.0m1.3m). The

improved cylinder-process for making glass sheets facilitated this. When the skele-

tal structure for building supports was developed, first through the use of cast iron,

then of wrought iron and later of steel and reinforced concrete [8], it became possible

for the entire external wall facades of buildings to be constructed of glass.

The invention of refrigeration systems to cool air was rapidly followed by their use

for the conditioning of air in buildings. Architects began to introduce this new tech-nology into their buildings (e.g. Frank Lloyd Wrights Larkin Building at Buffalo USA,

in 1904, was the first air-conditioned building). Large windows were employed in offices

and even residential buildings, which led to higher rates of solar-energy gains into

internal spaces and therefore a greater need for mechanical cooling. This concept was

exported and the American international style became especially popular after

World War 2 (19391945). Its rapid adoption was helped by the low unit prices of fossil

fuels and hence of electricity. So high energy-dissipating systems became increasingly

tolerated. This trend considered the building to be an enclosure that provides a ther-

mally-comfortable environment, which could be isolated from its hostile ambient-

surroundings. Thus, mechanical systems (e.g. refrigeration plant) were installed tomaintain the thermal comfort of the occupants of buildings, but at relatively high rates

of energy consumption, and with insufficient concern for the ambient environment.

Changes in habitation patterns occurred as a result of the introduction of this

international style. For instance, it led to the functions that occur within the various

internal spaces being chosen irrespective of the orientation of the building, its loca-

tion or the daily and seasonal changes of weather. The location of human activities

had previously been dictated by how well thermally-comfortable conditions were

achieved and maintained within the different rooms of vernacular buildings. How-

ever, thermal comfort is maintained in international style buildings primarily by

air conditioning to compensate for the high rates of solar gain that they incur.During the last 50 years, the use of double-glazing gradually became a feature of

contemporary architecture in order to help thermally isolate the interiors of buildings.


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Another characteristic, of contemporary windows, is the utilization of tinted and/orsurface-coated glasses [6,7] to reduce insolation transmission and glare. Excessive solar

gains in summer also have been inhibited by other methods, including the use of over-

hangs (see Fig. 2), reflective and light-coloured external surfaces, shade screens to fully

or partly obscure the windows and internal or external movable-shutters.

Nevertheless, there are, as yet no major trends to reduce the sizes of the windows

in the facades of new prestige buildings in the Middle East, or to re-adopt those

characteristics of the regions vernacular architecture that reduce the rates of the

heat gain or loss through the fabric of a building. Currently, in the Middle East, no

statutory standards exist which are designed to limit the rates of heat transfer

through the envelope elements of a building. Also, unfortunately much of the dataused in predicting the thermal performances of buildings is taken from manu-

facturers guides and codes of practice that emanate from foreign sources: foreign

architects may pay insufficient attention to the local climate and terrain and so

inappropriate, highly energy-wasteful buildings are constructed.

4. Window functions

Windows exist to permit daylight to enter the buildings interior, to allow venti-

lation and to provide views of the local ambient environment: psychologically theyare also desirable for contributing to the mental health of the occupants. Le Cor-

busier[9] stated that the history of architecture shows the struggle to admit daylight

Fig. 2. Overhang and shielding of a window to shade it from insolation: the vertical shield could be used

on either side of the window in order to shade it from the insolation.


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into the heavy habitual structures via windows. Architecture involves an aesthetic

interplay of light and form within building spaces in addition to ensuring sufficient

illumination to undertake visual tasks.

Daylight comprises both the unscattered direct-beam solar radiation and the dif-

fuse radiation from the sky, clouds and terrestrial surfaces in the line-of-sight from

the window. As highly-glazed buildings have become more common, the problemsof glare and severe overheating caused by excessive insolation entering their interior

spaces is increasingly recognised. Consequently, the search for techniques or systems

Table 1

The mean maximum and minimum temperatures for countries in the Middle East during 1996 [10] a

Month Day type Monthly mean maximum/minimum temperatures (in C) for the

considered location: (the altitude of the location in metres above

mean sea level is given in parentheses).

Egypt

(% 0)

Israel

(445)

Amman

Jordan

(764)

Damascus

Syria

(709)

Izmir

Turkey

(27.6)

Turkey

(1722)

January Sunny 23/5 13/5 12/4 12/2 13/4 -6/-18

Rainy 9 8 7 10 7

February Sunny 26/7 13/6 13/4 14/4 14/4 -4/-16

Rainy 11 8 6 8 7

March Sunny 29/11 18/8 16/6 18/6 17/6 -1/-11Rainy 5 4 2 7 7

April Sunny 35/16 23/10 23/9 24/9 21/9 10/-2

Rainy 3 3 3 5 9

May Sunny 40/21 27/14 28/14 29/13 26/13 17/3

Rainy 1 1 1 4 15

June Sunny 41/23 26/16 31/16 33/16 31/17 21/6

Rainy 2 12

July Sunny 41/24 31/17 32/18 36/18 33/21 25/9

Rainy 8

August Sunny 41/24 31/18 32/18 37/18 33/21 26/9

Rainy 1 7

September Sunny 40/22 29/17 31/17 33/16 29/17 22/4

Rainy 2 2 5

October Sunny 35/18 27/15 27/14 27/12 24/13 15/-7

Rainy 1 1 2 4 7

November Sunny 29/12 21/12 21/10 19/8 19/9 7/-5

Rainy 4 4 5 6 6

December Sunny 25/8 15/7 15/6 13/4 14/16 -2/-13

Rainy 7 5 5 10 7

a The rainy day description implies $9 mm of precipitation occurs whereas a sunny day implies

$8 h of clear-sky conditions occur, during the average day of the considered month


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that allow architectural freedom, while permitting the entry of adequate amounts of

daylight without causing discomfort, has intensified. The generally clear-sky condi-

tions and high rates of insolation found in the Middle East make achieving better

solutions to this design challenge vital.

5. Environmental considerations

5.1. Latitude

Most of the countries (e.g. Jordan, Iraq, Arabian Peninsula countries, Kuwait,

Egypt, Tunisia, Libya, southern Turkey, part of Iran, as well as the Gulf States),

considered in this report, lie between latitudes of 31and 37N, and experience

weather conditions characterised by high rates of insolation (see Table 1) [10]. This

shows the effect of rainy weather on the temperature variations experienced within

each month. The presence of rain infers that cloud cover exists and so an increased

reflection and absorption of insolation occurs in the higher atmosphere (i.e. espe-cially of the infrared waveband). This causes the indicated lower daytime tempera-

tures experienced.

Fig. 3. Thermal gain through the window facing south (for 21 June): (- - - -) shaded by the overhang; ()

unshaded [12].


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Due to the lack of both in-land fresh-water lakes and low rates of precipitation,

large areas of this region tend to be devoid of naturally-occurring green-foliage,

which could provide some shelter from exposure to excessive insolation.

5.2. Global insolation incident upon a horizontal surface

This parameter is dependent upon the latitude, the direct and diffuse solar-energy

intensities and the local humidity. Direct daily global radiation can reach a mean value

of Gmax of 6.8 kWh m2 in Jordan and 8.5 kWh m2 in Iraq and most of the Gulf

States. A sample of the solar conditions experienced in Jordan is presented in Table 2.

6. Insolation gains through vertical windows

Fig. 2 shows the components of some basic structures available to shade a win-

dow. Hamdan [12] measured the rates of energy gain or loss (including both diffuse

and direct components of the insolation) through vertical windows at Yarmouk,

Amman, Jordan (32 35 N, 35 37 E). He varied the magnitudes of some of the para-

meters defined in Fig. 2 to determine their effects on the rates of heat transfer

through the window. Fig. 3 shows the effect of using a horizontal overhang, wherethe ratio of the offset of the overhang above the glazing to the distance it protrudes

was 0.5. By undertaking tests on shaded and un-shaded windows, a reduction in

Table 2

Insolation at Baqura, Jordan (32 380 N, 35 370 E). The presented values ofG and Swere evaluated from

mean monthly data [11]

Horizontal plane: monthly mean value (19731976)

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean

Daily global radiation (kWh m2)

Gmean 2.7 3.4 4.3 5.3 6.4 6.8 6.6 6.1 5.3 4.2 3.0 2.9 4.8

Gmax 4.5 5.6 7.1 8.1 8.6 9.0 8.8 8.1 7.1 5.9 4.7 3.9 6.8

Gmin 1.2 1.6 2.2 2.6 3.9 7.1 7.6 7.0 5.6 3.1 1.4 1.1 3.7

G0 5.6 6.9 8.6 10 11 11 11 10 9.2 7.5 5.9 5.1 8.6

Ratio with respect to extraterrestrial radiation

Gmean/G0 0.5 0.5 0.5 0.53 0.58 0.59 0.58 058 0.58 0.56 0.52 0.56 0.55

Ktmax 0.75 0.75 0.76 0.76 0.75 0.78 0.77 0.74 0.73 0.72 0.74 0.74 0.75Ktmin 0.22 0.23 0.25 0.26 0.35 0.62 0.67 0.67 0.61 0.41 0.23 0.21 0.43

Mean daily diffuse radiation (kWh m2)

D estimated 1.2 1.3 1.8 2.1 2.0 1.8 1.9 1.7 1.5 1.2 1.1 1.1 1.6

D/Gmean 0.45 0.39 0.42 0.39 0.32 0.27 0.28 0.28 0.28 0.29 0.36 0.39 0.33

Daily sunshine duration (h)

S 4.9 6.5 6.7 8.0 10.7 11.9 11.2 11.9 9.9 8.6 6.5 5.3 8.5

Sd 10.1 10.9 11.8 12.8 13.7 14.1 13.9 13.2 12.3 11.2 10.4 9.9 12.0

So max 10.3 11.2 12.2 13.2 13.9 14.1 14.0 13.5 12.6 11.6 10.6 9.9


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Fig. 5 shows that the east and west-oriented windows tend to transmit more

heat during the day than those facing south or north for the considered location in

Jordan.

When using a similar size overhang for a west-facing shaded window as for the

south facing one (i.e. as for Fig. 3), the thermal gain was reduced by only 23%on the 21 December compared with 40% on 21 June. These percentages depend

upon the magnitudes of the glazing height H and the horizontal protrusion E of the

Fig. 6. Thermal gain through a vertical window, facing due west, 21 December: () unshaded; (- - - -)

shaded by a horizontal overhang and vertical shield [12].

Fig. 7. Rate of heat transfer through the vertical south-east facing window of the building (21 June): ()

unshaded; (- - - -) shaded by a horizontal overhang [12].


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vertical shield. As a result of hundreds of years of experience, most windows in

buildings in Amman tend to be oriented towards the southeast, and incorporate

protrusions (both horizontal and vertical) of the types shown in Fig. 2. The resultsillustrated in Figs. 69 are for the cases where the horizontal overhang and vertical

shield were present or absent for the various orientations.

Fig. 8. Thermal gain through a vertical window facing due east for 21 June: () unshaded, (- - - -) shaded

by a vertical shield and horizontal overhang [12].

Fig. 9. Thermal gain through a vertical window facing southwest, for 21 June: shaded by vertical shields

and a horizontal overhang as in Fig 2: () shaded, (- - - -) unshaded [12].


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Combinations of both large overhangs and/or protrusions were not considered

because it is unlikely that they would be employed in common practice.Figs. 5 and 8 show that significant solar gains occur through an east-facing win-

dow, especially during the morning. The rate of thermal gain reaches its peak about

Fig. 10. Intensities of direct, diffuse and global (i.e. direct+diffuse) radiation for 21 June 1996 at Kuwait

(latitude 24.9N).

Fig. 11. Variation of the transmissivity for clear glass of refractive index 1.53.


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8:00 am: because of the thermal inertia of the building structure and contents, the

maximum internal temperature ensues later in the day.When comparing the effects of shading for windows at different orientations, as

indicated in Figs. 79, it can be seen that the shadows cast by the overhangs and

Fig. 12. Variation of the effective transmissivity of vertical glazing to direct-beam solar radiation with

orientation and time of day for 21 June.

Fig. 13. Variation of the effective transmissivity of vertical glazing to direct-beam solar radiation for 21

March.


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vertical protrusions are more effective for the west and south orientations rather

than for the easterly ones. This is due to the lower solar altitudes for the easterly

orientation, which makes the two types of protrusion cast relatively smaller shadows

in this case.

7. Thermal-transmittance and daylighting behaviours of windows

There is a tendency to consider windows as apertures that allow solar radiation to

enter a building, so providing both daylight and beneficial heating of its interior.

This has arisen in part because of the popularity of passive-solar architecture in

Northern Europe and a desire for reducing the costs of heating buildings during cold

seasons, while ignoring summertime discomfort in them [13]. Consequently, much of

the development of glazing systems has concentrated primarily upon reducing the

rates of heat loss via them from the interiors of buildings.

The designs of the majority of highly-glazed buildings make little attempt to take

heed of facade orientation or the change of Sun angle with season. Glare and

excessive solar-gains from the east through to the west during the day occur as a

result in such buildings. In a relatively few cases, special coatings have been applied

to the glass in some buildings in order to reflect high proportions of the incident

insolation. However, these reduce the amount of solar energy transmitted in the

visible part of the spectrum as well as in the UV and the near-infrared wavebands.

For general daylighting purposes, a desirable system would have daylight providedby the diffuse component of the insolation, whilst almost totally excluding the

Fig. 14. Variations of the effective transmissivity of south-facing vertical clear-glazing to direct-beam

solar radiation with the day of the year.


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Fig. 15. Vertical glazed-windows to reduce the transmission of direct-beam insolation.

Fig. 16. Consequence of changing the inclination of a glass surface to the vertical upon the effective

transmissivity to direct-beam solar radiation for south-facing glazing.


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Fig. 17. Consequence of changing the inclination of a glass surface to the vertical for the effective trans-

missivity to direct-beam solar radiation for west-facing glazing.

Fig. 18. Steady-state heat-transfer behaviour of a vertical double-glazed window for a fixed temperature-

difference,T, between the glass panes [15].


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direct-beam radiation. However, the radiation received from the diffuse component

is much less than from the direct component (see Fig. 10).

Tilting the glazing can be used to control the intensity of the solar radiation which

is transmitted through a window. Tilting the glazing alters the angle of incidence ofdirect insolation impinging on its surface. Fig. 11 illustrates the change of reflectivity

of a typical glass surface with the angle of incidence: for angles of incidence

exceeding 60, the transmissivity is reduced significantly.

The variations of the transmissivity of vertical single-glazing to direct-beam

radiation with the time of day for several orientations, at latitude 32.63N, are

shown in Figs. 1214. The angle of incidence of the direct-beam radiation to the

glazed surfaces of a building changes with the time of day. For 21 June, the effective

transmissivity of south-facing glazing remains below 0.58 and the period of trans-

mission of solar irradiation only occurs from $09.00 to 15.00 h. As the vertical glass

faces an increasingly westerly direction, the transmissivity increases, from late

morning and during the afternoon, to values greater than those experienced by

south-facing vertical glazing. This occurs because the lower Sun angles (i.e. the solar

altitudes) in the afternoon result in relatively low angles of incidence of the direct-

beam solar radiation on these more westerly-orientated vertical glass-surfaces.

Fig. 19. Convective cell in a window cavity.


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A major change of behaviour can be observed in Fig. 13 for the south-facing

glazing where much higher transmissivities occur over a longer period of the day,

with the peak values being similar to those of the more westerly-orientated vertical

glazing. The behaviours of the more westerly-orientated windows are similar tothose of 21 June, but the transmission begins earlier in the morning for both the

S30W and S60W orientations.

On 21 December, when the solar altitudes are at their lowest in the Northern

Hemisphere, the transmissivities for south-facing surfaces are at their highest and

the daily period during which the window transmits is most prolonged see Fig. 14.

As summer approaches, the solar altitude increases: this causes both the effective

transmissivity and the period of transmission to decrease.

The diffuse component of solar radiation is much less directionally-dependent

than the direct-beam component and so will show less variations with the geometry

and orientation of the glazing system.

The proposed system to reduce solar gain uses a tilted-glazing element to increase

the angle of incidence of the direct-beam component during the day and conse-

Fig. 20. Schematic influence of inserts upon the main vortex in the cavity of a vertical double-glazed

window.


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quently decrease the effective transmissivity of the glazing to solar radiation. Envi-

saged vertical systems, each including a tilted element, are illustrated in Fig. 15; the

optimal value ofb being primarily dependent upon the latitude. Use of this optimal

angle would ensure the lowest energy transfer from the ambient to the indoorenvironment.

Systems such as that shown in Fig. 15(a) have been used previously, but are criti-

cised usually because of their appearance. The configuration shown in Fig. 15(b)

uses standard glazing-frames and a clear plastic film provides the reflecting element.

The thermal resistive behaviours of the cavities on either side of the transparent

membrane would also influence the thermal resistive behaviour of the glazed unit.

Fig. 21. The proposed triple-glazed unit.


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The consequences of altering the tilt of a single glass-pane from the vertical are

shown in Figs. 16 and 17 respectively. The effect of the glazings tilt angle is marked

for the south-facing glazing, with only a 10 glazing-angle from the vertical causing

a reduction of 6.6% in the peak effective transmissivity for day 80 of the year. Whenthe glazing tilt angle is increased to 30 from the vertical, then the peak effective

transmissivity is reduced by 72%. The behaviour of the effective transmissivity with

glazing angle is quite different for the west-facing window see Fig. 17. The onset

of the transmission of the direct beam solar radiation occurs later in the day as the

glazing angle is increased. The sharp cut-off of the lines at $18.00 h occurs because

this is the time of sunset.

8. Thermal insulation achieved via trapped air

Heat can be transferred through a window via radiation, convection and conduction.

In particular:

(i) direct and diffuse components of the solar radiation enter the building during

the day and heat is lost from the building by infra-red radiation emitted by the

glass surface;

(ii) a proportion of the insolation and the thermal radiation emitted by the

buildings interior surfaces and the ambient environment that are incident upon

the window system is absorbed by it and the glazing emits thermal radiation;(iii) convection occurs via the gas (usually air) in the cavities of a multiple-glazing

system; and

(iv) conduction occurs through the glass panes, the trapped air and window frame and

spacers that support and separate the glass panes of the multiple-glazing system [14].

The boundaries in multiple-glazing units inhibit convection in the cavities: the low

thermal conductivity of the trapped air contributes the major proportions of the

relatively high thermal resistances of these units [14]. This is achieved without for-

saking the advantage of the window as a means for providing natural daylight. The

use of partially-reflective coatings on the interior surfaces of the glass panes of multi-cavity glazing has been commonly employed [14].

The effective thermal transmissivities of the traditional vertical double-glazed sys-

tem were studied by Robinson and Powell using a guarded-hot-box apparatus [15].

For a small horizontal temperature-difference across a narrow, vertical air-filled

cavity at near-normal ambient temperatures, convective heat-transfer will occur if

the cavity is wider than 12 mm, and will rise in intensity, as the width is increased

(see Fig. 18). However, as the width of the cavity increases, the conduction con-

tribution to the rate of heat transfer through the cavity will decrease. Because these

two conflicting processes exist, an optimal cavity width occurs at which the thermal

resistance has a maximum value. For the range of air temperatures that typicalbuilding window-systems experience in the UK, the optimal cavity width is

approximately 19 mm, but would be slightly less in the Middle East (This optimal


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width decreases as the mean temperature of the two bounding vertical surfaces

increases.) The radiation view-factor for this system is almost unity for the range of

cavity widths considered and so the radiation contribution remains approximately

invariant for small temperature-differences between the vertical glass panes and doesnot influence significantly the optimal width [16].

Placing vertical inserts in the cavity shown in Fig. 19, so that it becomes as Fig. 20,

can deflect the vortex of the convective cell: x/W should have a value of approxi-

mately 0.25. This reduces the tendency to form localised relatively hot and cold

spots on the cold and hot surfaces respectively, and hence increases the thermal

resistance of the double-glazed system.

9. A proposed design

This is illustrated in Fig. 21 where a partially-reflective element bisects the air

cavity between the two vertical glass panes. The convective currents in the resulting

triangular-sectioned cavities affect significantly the thermal transmittance of the

glazed system.

The relative magnitudes of the convective and radiative components of the heat

transfer rate will be influenced by the angle of the transparent membrane and its

effective transmissivity. As is decreased, for a constant H, the cavities become

wider and the convective components of the heat transfer will rise. The optimal

values of, y and x to achieve maximum thermal resistance have to be determinedfor each proposed application, location, and the values of H and W chosen for the

design.

Such a system would allow considerable daylight to enter through the window via

the diffuse component of the solar radiation, while inhibiting heat gains. Addition-

ally the units would have different thermal resistances according to whether the

heat transfer is from the left to the right, or vice versa, for the configuration shown

in Fig. 21. Hence in terms thermal insulation, it would be beneficial to be able to

rotate such a unit about a central vertical axis of the window within its frame,

according to whether it is day or night, summer or winter, or the imposed weather

conditions.

10. Conclusions

. Experimental data regarding the thermal behaviours of the large windows now

prevalent in modern buildings in the Middle East are rare.

. Modern designs of glazing systems tend to ignore the energy-thrift lessons

provided by the vernacular architecture of the region.

. A new triple-glazed system has been proposed that will reduce the transmissionof direct-beam solar irradiation and the heat gains from the ambient environ-

ment, while maintaining adequate levels of daylight in the buildings interior.


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References

[1] Kob W, Binder K, Baschnager J, Paul W. Glass physics: still not transparent. Physics World

1999;12(12):P54.[2] Volf M. Models in the chemistry and technology of glass. In: Mathematical approach to Glass.

Kavallier, Sazava, Czechoslovakia, Elsevier 1988, p. 1939.

[3] Lloyd-Wright F. Evolvement of the city. In: Collected writings The living city, 1958, Vol. 3.

Barnes and Noble, Minnesota, USA, 1994. p. 250265.

[4] Fletcher B. The ancient near East: Mesopotamian civilization. In: A History of Architecture, 20th ed.

Dan Cruickshank, USA, 1996.

[5] Fatthi I, Warren J. Environmental considerations. In: Traditional houses in Baghdad. Coach Pub-

lishing. Horsham, UK, 1982. p. 20, 4044, and 60.

[6] Web-site by PPG Industries Inc. for Glass Reinforcement for Thermoplastics. History of glass.

Pittsburgh, PA, 15272 USA, 1998.

[7] Web-site by AGSA, history of tinted glass. Art Glass Suppliers Association International; USA,

Zanesville, OH, 1998.[8] Button D, Pye B. The building structure. In: Glass in building. Butterworth-Heinemann, Oxford,

1994. p. 410.

[9] McGrath R. Glass in architecture and decoration. London: Architectural Press, 1961.

[10] http://www.ceedex.co.uk/outerlimits/info/weather/meast.html, Weather forecast for the Middle East,

CNN search engine, August 1998.

[11] Palz W, Greif J. European solar atlas radiation on horizontal and inclined surfaces. Commission of

the European Community, Brussels, 1996 p. 1505.

[12] Hamdan MA. Thermal gains through windows. In: Energy conservation, Vol. 35, No.6, Elsevier

Science, UK, 1994. p. 5016.

[13] Loxom F. Meteorological data for passive-solar design. collection of reports for ETSU model-

refinement study, Environment Dept., Central London Polytechnic, 198586.

[14] Griffiths PW, Norton B. Detailed simulation of heat transfers across planar evacuated glazing.Building Materials Magazine 1996;24(3):1427.

[15] Kusuda T. Heat transfer in buildings. In: Rohsenow WM, Hartnett JP, Ganic EM, editors. Hand-

book of heat-transfer applications, 2nd ed. McGraw-Hill Book Company, 1985. p. 22040.

[16] Rubin M, Von Rottkay K, Powles R. Window optics. Solar Energy 1998;62(3):14960.


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